1. Field of the Invention
The present invention relates to a film used in wrapping materials that is required to be airtight such as wrapping materials for food, medicine and electronic components, the wrapping materials having excellent gas barrier and retorting properties and bending resistance (gelbo characteristics).
2. Description of the Related Art
As a conventional gas barrier film, the following films are known: films comprising a plastic film coated with a vinylidene chloride copolymer or a copolymer of ethylene and vinyl alcohol; and films comprising a plastic film having a thin film of aluminum or an oxide (such as silicon oxide and aluminum oxide) laminated thereon.
However, the former films have an insufficient gas barrier property against vapor, oxygen and the like. Particularly, when such films are used at a high temperature, the gas barrier property is markedly low. Moreover, the films comprising the vinylidene chloride copolymer have a problem of affecting the environment since they generate chlorine gas when incinerated.
Among the latter films, those having a thin film of aluminum are economical and have an excellent gas barrier property. However, since the film is opaque, the contents therein can not be seen from the outside. Moreover, when the film is used for wrapping retort food, it can not be cooked in an electronic oven since micro waves do not penetrate the film.
U.S. Pat. No. 3,442,686 describes a transparent flexible gas barrier film comprising an inorganic thin film such as a film of silicon oxide and aluminum oxide evaporated on a surface of a synthetic resin film substrate and a film further comprising another resin film laminated on the gas barrier film.
Japanese Patent Publication No. 51-48511 discloses a gas barrier film comprising Si.sub.x O.sub.y (for example, SiO.sub.2) evaporated on a surface of a synthetic resin. When such a silicon oxide type film is used for wrapping, the contents therein can be seen from the outside since it is transparent. Such a film can be used in an electronic oven.
But the above described films have only an insufficient gas barrier property. Among such films, one having a particular composition, that is, one having a SiO.sub.y (wherein y=1.3 to 1.8) type thin film has a relatively excellent gas barrier property. However, it has a brownish color. In addition, the gas barrier property and the bending resistance of such a silicon oxide type film are still insufficient. "Having a bending resistance" means not losing a gas barrier property by bending a film in steps after forming a film. (for example, steps of laminating, printing and making bags) or by careless handling of the film. The bending resistance is evaluated by measuring the gas barrier property after the gelbo treatment.
Japanese Laid-Open Patent Publication No. 62-101428 discloses a gas barrier film comprising a thin film containing aluminum oxide as a main component. This film is completely transparent and chemically stable, and the material costs are small. However, the gas barrier property against oxygen and bending resistance are not sufficient. Moreover, the film requires careful handling because the thin film itself is brittle.
Japanese Laid-Open Patent Publication No. 2-194944 discloses an Al.sub.2 O.sub.3 --SiO.sub.2 type film as a gas barrier film with a retort property. "Having a retort property" means having an excellent airtightness and flexibility even after a high temperature treatment. The retort property is evaluated, for example, by measuring the gas barrier property under a condition for producing retort food. The Al.sub.2 O.sub.3 --SiO.sub.2 type film comprises an Al.sub.2 O.sub.3 layer and a SiO.sub.2 layer laminated successively on a plastic film. A large scale machinery is required for the production of such a film. The gas barrier property and bending resistance of such a film are not sufficient either.
Further, a gas barrier film comprising a layer made of a composite oxide (an oxide containing two or more kinds of metals) formed on a film substrate is known. For example, Japanese Laid-Open Patent Publication No. 1-176069 discloses an M--Si--O type gas barrier film, wherein M is at least one kind selected from the group consisting of Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ag, In, Sn, Sb and V. Japanese Laid-Open Patent Publication Nos. 61-297134 and 61-297137 disclose a MgO--SiO.sub.2 type and a MgO--CaO type gas barrier films, respectively. However, none of these films has a sufficient gas barrier property, an excellent bending resistance and a retorting property.
The thin film formed on the plastic film must be as thin as possible in order to improve the bending resistance and, the contrary, the thin film must have a certain thickness (for example, 2000 angstroms) so as to improve the retort property. When a film comprising an aluminum oxide thin film laminated on a plastic film is used for wrapping retort food, a range of a preferable thickness of the film is particularly small. Moreover, the film used for wrapping retort food is required to be resistant against conditions of the various steps of production such as the heat sealing step, a printing step and the like when it is made into a bag.
As described above, a transparent gas barrier film with satisfactory gas barrier and retort properties and an excellent bending resistance has not been provided.